1. Field of the Invention
This invention relates to a variable spectrum element.
2. Description of the Related Art
Variable spectrum elements each including an etalon device and a control unit have been known up to now as a variable spectrum element in which one of or both of a pair of optical substrates arranged opposite at a distance from each other are moved by an actuator like a piezoelectric device so that a distance between the surfaces of the optical substrates opposite to each other or a distance between the reflective films formed on the surfaces of the optical substrates opposite to each other (which is collectively called “surface distance between the optical substrates” hereinafter) is changed, with the result that optical characteristics of the variable spectrum element can be varied. (For example, refer to Japanese Patent TOKUKAI NO. 2008-129149.)
Also, it is known that such variable spectrum elements include a variable spectrum element in which: capacitive sensors for measuring a surface distance between the optical substrates are arranged on the surfaces of the optical substrates opposite to each other respectively in order to make the optical substrates have a desired surface distance; the current surface distance between the optical substrates is measured by the capacitive sensors with a predetermined sampling period; the measured surface distance between the optical substrates is compared with a desired surface distance between the optical substrates; and an actuator is made to operate on the basis of the result of the comparison between the measured surface distance and the desired surface distance so that the surface distance between the optical substrates is adjusted. (For example, refer to Japanese Patent TOKUKAI NO. Hei 6-241899.)